This invention relates to stepper motor drive assemblies in general, and specifically to a positively locking brake for such an assembly.
The air flow control valves used automotive air conditioning and ventilation systems have conventionally been swinging door type valves, turned back and forth over less than a full turn by a conventional electric motors. A conventional electric motor has an inherent resistance to being back driven. Furthermore, to step down the speed and elevate the effective torque of these motors, reduction gears are typically used, which are inherently difficult to back drive. The combination of motor and reduction gear resistances generally provide sufficient holding force to keep the valves in place at any given position, when the motor is turned off.
Swinging door type valves are, in some applications, being replaced by so called film valves, in which a roll of flexible film with vent openings is rolled back and forth between rollers, somewhat like a window shade with a roller at both the top and bottom. Typically, one roller will be powered and turned, while the other is spring loaded to take up, or wind out, the film roll, maintaining the belt in tension. Instead of conventional motors, stepper motors may be used to advance and wind up a film valve, since they provide the potential for precise control of the film position. An electric stepper motor comprises a primary member and a secondary member which move relative to each other. The primary member, usually the stator, is wound with a plurality of regularly angularly spaced drive coils having central iron cores which energized by drive pulses in some switched sequence. The secondary member, usually the rotor, is axially and radially centered within the stator, supported for rotation by suitable bearings. Typically, the rotor consists of two toothed discs of magnetic material separated by a permanent magnet between them. As the drive pulses are switched to different drive coils, the teeth on the rotor discs move to maintain alignment with the magnetic axes of those coils which are energized and thus relative rotation between the primary and secondary members takes place in a precise manner. When the stator drive coils are de energized, the rotor""s magnetic poles still tend to take on a stable, pre determined angular position within the stator, with the teeth of the rotor aligned with the stator coils. This stable position, while predictable, is not a strongly maintained, and does not strongly resist turning of the rotor.
While it is precisely controllable, and efficient, a stepper motor generally is slower than a conventional electric motor by a factor of ten, and proportionately easier to back drive. Film valves must move through multiple turns to be effective, unlike flapper door valves, and any reduction gears used to step down the already lower motor speed will have a much lower step down ratio. The net effect, then, with a stepper motor and its reduction gears is a far smaller inherent resistance to back driving when the motor is de energized, lower by as much as a factor of 100. The film valve tension spring can even potentially over power the stepper motor assembly when it is turned off, and pull the film valve out of its desired position. Therefore, some other means of holding the film valve in position when the motor is de energized may be needed.
The subject invention provides a means of positively holding or braking a stepper motor rotor relative to the stator, automatically, when the motor is de energized, and releasing it automatically when the motor is re energized. Any mechanism driven by the stepper motor is therefore also braked and held when the motor is de energized.
In the preferred embodiment disclosed, a stepper motor has a conventional stator, but the rotor is co axially supported within the stator by a central shaft by bearings that allow the rotor to shift to one side, out of axial alignment with the stator. A constantly acting resilient means, such as a spring, tends to push the rotor in one axial direction, out of its axial centered position within the stator. When the stator is energized, however, its electro magnetic force is sufficient to overcome the mechanical resilient force, and pull and keep the rotor axially centered within the stator. The resilient means can therefore actually shift the rotor to the side only when the stator is de energized. A toothed locking ring of non magnetic material is provided, fixed relative to the stator, that registers with the teeth of the rotor so as to lockingly engage with them when the rotor shifts to the side at the time of stator de energization. This is possible because of the pre determined, stable angular position that the rotor teeth take on relative to the stator (and locking ring) when the stator is de energized. The rotor is thereby positively locked and prevented from turning, and any load or mechanism driven by the motor is thereby held stationary, as well. When the stator is re energized, the pulling of the rotor back to center within the stator disengages and frees the rotor, also automatically.